Which? wants supermarkets to come clean with their customers over special offers. Photograph: Robert Brook/Alamy

An investigation into thousands of individual supermarket prices has revealed widespread "misleading" pricing tactics which are duping shoppers into believing they are getting a better deal than they are.

In its largest look at special offers, researchers from the consumer group Which? trawled through more than 700,000 supermarket prices over the course of a year looking at both discounts and multibuys. They found dubious offers that affect everything from pet food and fresh fruit to toiletries and wine.

Using data from independent grocery shopping website MySupermarket.co.uk from 31 January 2011 to 1 February 2012, the researchers identified four main tactics designed to make bargains look unmissable.

Firstly, products which become more expensive than the original price per item when they go into a multibuy offer. They cited an example in which Asda doubled the price of a single Müller yogurt from 30p to 61p as they went on to multibuy at 10 for £4. The price went back to 30p when the offer ended.

Secondly, Which? singled out products being sold on "offer" when in fact they had been on sale at the reduced price for longer than the original price. For example, Tesco sold Beck's beer for 190 days on discount and only 70 days at the higher price.

Thirdly, it identified prices being increased immediately before going on "offer" making the discount appear more significant. For example, Ocado strawberries increased from £3.89 to £4.38 for 13 days. They were then sold as "was £4.38 now £2.19/£2.29/£2.25" for 112 days, although there were nine days within this period when they were sold at £4.38.

Finally, Which? discovered products that were not sold at what was claimed to be a "higher" price immediately before going on offer. For instance, Aquafresh Milk Teeth Toothpaste was labelled as "was £1.74, now £1.15" at Asda. The highest price it was sold at before the offer was £1.17.

Which? is urging the government to tighten up its rules on pricing to give more protection to consumers. While the existing rules are supposed to make sure that supermarket special offers do not mislead shoppers, it feels they leave too many loopholes. Richard Lloyd, executive director of Which?, said: "It's unacceptable that shoppers are confused into thinking they're getting a good deal when that might not be the case.

"Consumers should not have to worry about whether a special offer is really 'special', so we want the supermarkets to play fair and the government to tighten up pricing guidelines so that people can shop with confidence."

But the stores claimed that the pricing anomalies were simply mistakes.

Tesco said in a statement: "We change millions of price labels in store and online each week and we sometimes make mistakes, for which we apologise. We make every effort to ensure we act in accordance with government guidelines on price promotions."

Asda said: "We are only human, and occasionally we make mistakes. By and large our systems and procedures ensure those instances are kept to an absolute minimum, but when we do get it wrong, we put our hands up to say sorry, and put things right as quickly as possible."

An Ocado spokesman described the example highlighted by Which? as an "isolated occasion".

About this article

This article was published on
the Guardian website
at 04.09 EDT on Thursday 24 May 2012.
A version appeared on p2 of the Money section of the Guardian
on Friday 25 May 2012.
It was last modified at 22.01 EDT on Tuesday 20 May 2014.
It was first published at 01.30 EDT on Thursday 24 May 2012.